1. Technical Field
The present invention relates to a toric contact lens, and particularly relates to the toric contact lens and a method for manufacturing the same, capable of stabilizing a directivity of a lens (preventing rotation of the lens) on a cornea during wearing the lens.
2. Description of Related Art
Usually, astigmatism indicates a state that lights emitted from one point of an outer field are not converged to one point in an eye, due to a different curvature radius between a vertical direction and a horizontal direction of the eye, when a shape of a cornea or a crystalline lens is not a clear spherical surface, and a toric contact lens is generally known as a soft contact lens for correcting the astigmatism.
The toric contact lens needs to be always worn in a direction determined for the eyeball in terms of a nature of the astigmatism. Namely, a portion intended to be a top (upper end portion) of the lens must be always positioned at a top (upper end portion) of a wearer. For example, a visual acuity becomes easily unstable if a contact lens is rotated by a blink (closing action of an eyelid) during wearing to thereby deviate an astigmatic axis of the toric contact lens. Therefore, a method for preventing a rotation of the toric contact lens or stabilizing an axis, is employed. As such a method, methods such as prism ballast and slab-off are given as examples.
As an example of employing the prism ballast, patent document 1 discloses a toric contact lens having a prism ballast structure which is formed so that a thickness is increased toward a lens lower end along an up and down lines parallel to a vertical meridian. In this example, the thickness is designed to be substantially uniform in a horizontal direction vertical to the vertical meridian. With this structure, rotation of the lens is prevented and a wearing feeling is improved. Further, as an example of employing the slab-off, patent document 2 discloses a toric contact lens with a flat portion provided on a peripheral part of a lens, having a mirror image symmetry with respect to a horizontal axis passing through a middle point of the lens, in such a manner that a portion of a vertical axis orthogonal to a horizontal axis is formed to be thinnest, and the thickness is gradually increased along the flat portion to both sides from the vertical axis, so that a horizontal axis portion is set as a maximum thickness portion, to thereby achieve a dynamic stability of the axis.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2004-506925
Patent document 2: unexamined utility model application publication No. 1975-133151
Incidentally, the toric contact lens disclosed in the aforementioned patent document 1 has the prism ballast structure in which the thickness is increased from an upper end to a lower end in an outside zone, with a difference of the thickness being remarkable from a lower end portion to a lower side edge in an inside zone. As the thickness of the lower end portion of the toric contact lens is increased, rotation of the lens occurs easily every time a wearer blinks when the toric contact lens is worn by a person whose lower portion of a cornea is swelled, and directivity of the lens becomes unstable, thus making it difficult to achieve good axis stability and making it impossible to sufficiently satisfy with a wearing feeling. Further, in the toric contact lens disclosed in the aforementioned patent document 2, although good wearing feeling can be obtained, the lens is rotated on the cornea every time the wearer blinks, because a center balance of the lens exists at a geometric center, thus making it difficult to obtain the axis stability.